Down syndrome (DS) results from triplication of human chromosome 21 (chr21) and is the most common genetic cause of mental retardation. It is hypothesized that DS is initiated by increased expression of genes located on the triplicated chr21, causing abnormal brain development in DS, but also resulting in further neurological complications, prominently Alzheimer's disease (AD)-like dementia, as DS patients grow into adulthood. By a number of neurological and neuropathological criteria, the DS population is at substantially elevated risk for AD-like neurodegeneration-and by at least 20-30 years earlier than the general population. In conducting a hypothesis-neutral screen of genes located on the DS trisomy for enhancement of amyloid precursor protein (APP)-induced neurodegeneration, we found that one of these genes, the - secretase enzyme BACE2, appears to enhance the rate/extent of neuronal degeneration in a brain slice-based model of AD. Based on previous reports in the literature and our preliminary studies, we hypothesize that BACE2 cleavage of APP generates a truncated version of -amyloid (A) that may not be amyloidogenic but nevertheless drives significant neurodegenerative processes even in the absence of full-length A. In order to rigorously test this hypothesis, it will be necessary to directly identify and characterize this presumptive A fragment generated by BACE2. However, currently available immunoreagents are not suitable for this purpose. Thus, the goal of this research proposal is to develop and test novel antibodies directed at the presumptive BACE2 A fragment; to use these reagents to demonstrate that a truncated form of A is produced by BACE2 cleavage of APP; and finally to isolate and directly sequence the BACE2-truncated A fragment. The specific aims are thus to: Specific Aim 1: Develop an antibody to the C-terminal half of the canonical A sequence. Specific Aim 2: Directly MS sequence the A fragment produced by BACE2 cleavage. If successful, this research strategy will provide further evidence and the tools necessary for testing the hypothesis that BACE2 could be a favorable potential drug target for slowing/preventing the progression of AD-like neurodegeneration in DS. Moreover, given that BACE2 is also expressed in the brains of the general population, albeit at lower levels, such BACE2-targeted drugs may also be important to investigate in the context of sporadic and familial AD, as the highly-selective BACE1 inhibitor drugs being developed today could unmask neurodegenerative processes mediated by BACE2 that may be latent in the general population as well. PUBLIC HEALTH RELEVANCE: Down syndrome (DS) results from the triplication of human chromosome 21 (chr21) and, with an incidence of 1 in ~750 births, is the most common genetic cause of mental retardation. Tragically, as DS patients grow into adulthood, they suffer from further neurological complications, most notably Alzheimer's disease (AD)-like dementia - at present, there are no drugs available to patients that can slow or halt the progression of early-onset AD in DS. There is thus an urgent need to understand the underlying molecular genetic mechanisms that lead to AD in DS in order to support the rational design of new therapeutics to combat AD in DS. In this context, the present proposal seeks to develop key immunoreagents to enable testing the hypothesis that one of the DS trisomy genes, BACE2, may be an important drug target candidate for the treatment of AD in DS, and potentially for treating AD in the general population as well.